How slippery the tires are, when the car is sliding. Smaller value means smaller friction. You could put bigger values to bigger wheels, but not too much. Friction angles also affect to friction, but I'll try to explain them after this. The surface affects to friction too: the surface materials are defined in tracks' textfiles, and usually the tyre road friction is 1.0 there. (Default value in C2, but actually it should be 0.75 for dry asphalt, like in real life.) But it doesn't matter, since you can put the 0.75 to tyre's friction, if you want. The result will be the same. Heavier cars should slide more than light cars because of their mass, so you should put smaller values to bigger cars.
0.2 = Greased tyres. (the powerup)
0.8 = Normal (realistic) value, but not for people, who just want to have fun and drive fast.
1.0 = Default value for cars. A bit too much friction, but still a good value.
1.5 = For fun, expect that high values make the cars flip over in turns if they have to high centre of mass.
Traction fractional multiplier
This affects to acceleration, and how well your car can climb up a slope. If put a big number here, and rev the engine to full RPMs while holding the handbrake on, you'll get a "rocket-start" when you release the handbrake.
0.5 = The wheels is either very small or in bad condition. Only little grip.
0.8 = A bit slippery, but it's OK.
1.1 = A good default value.
1.4 = A sports car's tires. You will get a fast acceleration, if the engine doesn't limit it.
2.0 = Very thick tires or what?
3.0 = Fun setting. (You'll definitely get those "rocket-starts" now.)
I don't know how I should explain this, but I know good values for this, so I'll just tell you them.
But first let me tell you that I use a steering wheel, and these might not be good ones for keyboard users.
Sometimes maybe, but not always. Some cars which I have edited handle well with both steering wheel and keyboard,
but some cars don't: they have too slow steering.
Front wheels: 81 , 76 Rear wheels: 76 , 81
That gives a kind of the same steering like in NFS Porsche 2000, I think. If you increase the difference between these two values, the car will steer more quickly.
If you put smaller number to both of the them, the friction will decrease, and it takes a longer time to stop the car. (Except that if you just use the brakes.)
Bigger numbers will increase the friction, and the car stops faster. You could try smaller values for bigger cars.